On evaluating the dynamic characteristics of an amplifier or the like, its settling characteristics, such as the settling time of its step response, are important.
In the prior art, measurement of the settling characteristics is done by directly observing the output signal of the "device under test" (or DUT), with an oscilloscope, a high speed digitizer, or the like. In order to avoid degradation of the performance of the measurement system, caused by saturation of the input amplifier of the oscilloscope used for the measurement, some measurement systems have employed a differential analog operation. An example of such a measurement method is described by the article entitled "Settling-time measurements demand precise test circuitry," by Jim Williams, EDN Nov. 15, 1984.
When highly accurate settling characteristics measurement is attempted by such a method, measurement error is introduced from the transmission characteristics of the transmission line connected from the device under test or DUT and the characteristics of the differential amplifier With respect to the waveform distortion caused by the transmission characteristics of the transmission line, see, for example, page 206 of Electronics Series 44, "Pulse Measurement and Its Applications" published by Sampou Co. Ltd. in Japan. It is quite difficult not only to compensate for this distortion but also to evaluate the distortion quantitatively. It is also difficult to obtain an amplifier having performance good enough to be used for the measurement system for the settling characteristics.
Furthermore, when the measurement data is obtained by sampling and digitizing the output signal of a DUT, the cables, probe amplifier, and sampler introduce error. Though such an error component, which is caused by the cable and probe amplifier, could be eliminated by connecting the output of the DUT directly to the sampler without any intermediate devices, the error caused by the sampler would still remain. In addition, because such an arrangement of connections is often impossible, in view of the construction of the measurement system, the full arrangement would lose universality.
A major object of the present invention is to provide a novel method for measuring settling characteristics with high accuracy and without the drawbacks of the prior art.